Fuel cells have proven to be an important technology in improving energy efficiency. Although a mature technology, recent advancements have spurred fuel cell development for commercial applications. As the debate continues over the future energy supply, there exists a strong interest in improving the efficiency of current fuel sources. The efficient use of current fuels such as gasoline, diesel, or methane through fuel cells can bring fuel cells into the forefront of helping ease the energy crisis.
Generally, fuel cells cannot directly convert liquid hydrocarbon fuels into energy. Instead, liquid fuels, such as gasoline and diesel, need to be first converted by a fuel reformer into hydrogen and carbon monoxide as the fuel gas for fuel cell. The resulting gas (reformate) is fed into the anode of a fuel cell and electrocatalytically converted into water, carbon dioxide, and electricity. Typical methods of reforming include partial oxidation reforming (POX), autothermal reforming (ATR) and steam reforming (SR). Reforming hydrocarbon fuels containing polyaromatic hydrocarbons and sulfur-containing polyaromatics such as those found in commercial diesel is a difficult process. Incomplete reforming causes coke buildup, a solid carbonaceous residue, within the fuel cell due to condensation of unconverted hydrocarbons leaking from the reformer. Coke buildup blocks the reformate from reaching to the anode of the fuel cell, hindering fuel cell performance. See Fuel Cell Handbook, 7th ed. National Energy Technology Laboratory, 2004 (DOE/NETL-2004/1206) (OSTI ID: 834188) pg 8-11 2004, herein fully incorporated by reference. Coke buildup is the result of sulfur content in the fuel, the use of a heavy aromatic fuel, or a combination thereof.
Coke buildup is a major problem with solid oxide fuel cells (SOFC), which operate at temperatures especially susceptible to coke buildup. Generally, coke buildup forms downstream from the fuel reformer at the entrance of the SOFC, where the temperature is usually lower than that of the fuel reformer.
As an example, in a fuel cell system utilizing diesel fuel it is common to use an autothermal reformer, and a SOFC. The fuel reformer typically operates in excess of about 600° C., preferably 700-800° C., to process the diesel into a hydrogen rich reformate. Unconverted and fragmented hydrocarbons are generated in the reformer as a byproduct of processing the diesel, as diesel has both high heavy aromatics and sulfur content. When carbon in the reformate reaches the anode of the fuel cell, it cools within the window of carbon condensation (generally in the range about 400° C. to 650° C.), and condenses onto the fuel cell inlet or on the surface of anode, creating a coke buildup. The coke buildup blocks the reformate from reaching the electrocatalyst inside of the fuel cell and stops the electrochemical reaction for generating electricity. As more coke buildup occurs, the fuel cell becomes less efficient at generating electricity.
One method of reducing coke buildup is to increase the operating temperature of the fuel cell, as well as heating any channels between the fuel reformer and the fuel cell. Increasing the temperature of the channels and the fuel cell greatly reduces the energy efficiency of the fuel cell system due to the continuous heating required to maintain the operating temperatures close to 1000° C. Furthermore, as the result of reformate distribution and uneven reaction rate, the fuel cell itself does not normally heat uniformly causing cooler spots, on which coke condenses. These cooler spots with cumulated coke buildup lead to the reduction of the catalytic capacity of the fuel cell until the fuel cell is cleaned.
Another method of removing coke buildup is to disassemble the area of the fuel cell with the coke buildup and physically clean any coke buildup. Disassembly requires the fuel cell to be shutdown, which interrupts the power production. The anode of the fuel cell may also be adversely affected by ambient air exposure during the cleaning process. For example, the commonly used reduced nickel anode in a SOFC oxidizes in ambient air to form nickel oxide, causing thermal and mechanical stress in the fuel cell anode, which eventually destroys the SOFC.
Yet another method of removing coke buildup is flushing the reformate channel and the fuel cell with oxidizing reagents such as oxygen at elevated temperatures. The oxidizing reagent reacts with the coke buildup and forms a carbon dioxide gas, which easily passes through the fuel cell system. Since the oxidizing reagent also reacts with hydrogen, obviously the fuel cell must be shutdown during the cleaning. Furthermore, oxidation of the anode of the fuel cell (commonly reduced nickel) will also occur if the oxidizing reagent reaches the anode, thus poses a risk of destroying SOFC.
Therefore, a need exists to have a fuel cell system that is capable of continual and uninterrupted processing of fuels having heavy aromatics, high sulfur content, or combinations thereof without coke buildup to impair the fuel cell operation.